Update CSS class identifier normalization

Hi! Thank you all for your work on docutils!

I've recently started using Plausible for analytics tracking. Unfortunately
their naming schema for custom events is incompatible with docutils CSS
class normalization, since Plausible relies on either "=" or "--" for
naming custom events (plausible docs
<https://plausible.io/docs/custom-event-goals#you-can-also-add-class-names-directly-in-html>
).

Reading through the rationale for the chosen normalization
<https://docutils.sourceforge.io/docs/ref/rst/directives.html#identifier-normalization>,
why is "=" excluded? Would an update ever be considered where "=" was
allowed in class names for docutils?

Thanks!

- Sarah